1. Field of the Invention:
This invention relates to an end mill with a plurality of curved, indexable cutting inserts and particularly to a novel means for securing the inserts.
2. Background of the Invention:
This Application is copending with application Ser. No. 833,372 filed Feb. 25, 1986 now U.S. Pat. No. 4,681,485 and is commonly assigned to GTE Valenite.
An end mill is a cutting tool which includes a cylindrical body configuration having a shank portion and a cutting portion. The cutting portion contains a plurality of helically disposed teeth or on-edge cutting inserts extending from the shank end to the opposite, free end of the tool, which, when rotated and brought into contact with a workpiece, can remove metal from the workpiece.
The problem inherent in the use of such a conventional end milling cutter manifest themselves in excessive wear and relatively poor cutting action, or both, of the cutting tool, owing to the fact that the entire length of a cutting edge may be applied to the workpiece at the same time. Further, chips are continuously produced at the work area which, if not satisfactorily removed from the work area, require excess power to be applied to the tool to effect cutting while also resulting in increased wear of the cutting edges. There have been many attempts made to improve the cutting action and decrease the wear in such tools.
For example, when an end mill is provided with a neutral (zero) axial rake angle for its cutting edges, it can move through metal leaving a reasonably straight wall. By axial rake angle of the cutting edge is meant the angle substended by extending the cutting edge until it meets the longitudinal axis of the tool. If the angle is less than 90.degree., it is considered to be a positive axial rake angle while if this angle is 0.degree. or the edge is parallel to the axis, the axial rake is neutral. There are certain disadvantages to conventional, neutral (zero) axial rake cutters. The chips cut from the workpiece do not discharge readily and are apt to dull the tool as they are pulled back into the groove or slot being cut. Thus, in most cases, it is desirable to have a cutting action which is obtainable with a positive axial rake angle. The positive axial rake cutting tends to move the chips up and away from the cutting area, and the very nature of the positive rake cut makes for a smoother cutting action and prolonged tool life. In addition to the enhancement of chip removal, by use of an axial rake angle in an end mill cutter, excessive wear and its corollary of relatively poor cutting action can be reduced extensively due to preclusion of the entire length of the cutting edge not being applied to the workpiece at the same time, but rather in a progressive cutting or shearing step.
Further, when the active cutting edge is disposed not only at a positive axial rake angle, but also a positive radial (i.e., double positive) rake angle, the cutting edge is maintained relatively sharp and less power is required to cut the workpiece, as compared to the case when the edge is at a negative radial rake and is thus relatively blunt as it strikes the metal. By a positive radial rake angle is meant the angle substended by an extension of the cutting face at the cutting edge and a radial line passing through the cutting edge in a plane perpendicular to the longitudinal axis of the tool which is less than 180.degree.. Thus, an on-edge insert for a cutting tool having cutting edges adapted to cut at double positive rake angles is advantageous in that the active cutting edge can cut the work with relatively low power, yet can withstand comparatively high forces before failing. Further, chip removal is greatly enhanced thereby minimizing chatter and vibration of the tool as it removes metal from the workpiece.
While provising a mill end cutter with on-edge inserts having both a positive axial and radial rake angle will enhance the cutting of a metal workpiece, I have found that the cutting action can be improved further by providing cutting inserts which are indexable to present rounded cutting edges to the workpiece. Particularly, the lower-most cutting insert for initial contact with the workpiece is preferably in the shape of a rhomboid and is provided with opposed, unique curved cutting edges, each having its radial center below the center of the insert itself which, as it cuts, will cause a chip cut from the workpiece to be pulled up and away from the workpiece during the cutting motion, as well as away from the workpiece because of its axial rake. The curved cutting edge of the rhomboid insert can be defined as an arc presenting a zero or neutral to positive radial rake angle throughout most of its cutting edge in contact with the metal workpiece as it cuts through the workpiece. The rounded edge significantly increases the wear and life of the tool before the insert has to be indexed because of its progressive (shearing) cutting action as compared to a straight cutting edge in contact throughout with the workpiece and, as indicated, the curved edge serves to push the chips away from the workpiece into an appropriate helical "flute" or slot adjacent the cutting edge much more readily where it can be carried away from the workpiece. Further, the tool runs much quieter because of the shearing action provided by the "rounded" edge as it progressively enters the workpiece as compared with a "straight" cutting edge insert which provides for simultaneous cutting action all along the workpiece and cutting edge. Finally, the insert has a lead at the end of the curve or arc, which, because of its radius, results in a high strength corner. The tool design thus results in a very strong tool which highly resists chipping, thermal cracking, or chip adhesion.
The inserts can be mounted on a 20.degree. helix which results in excellent chip expulsion into an adjacent flute.
The rhomboid insert can be used in combination with a plurality of round or circular indexable inserts which are very economical since they can be indexed to present a plurality of cutting edges as the insert wears in combination with the longer-life rhomboid inserts which are used to effect initial cutting and entry of the workpiece at the bottom end of the tool. As with the rhomboid insert, the round inserts can be provided with a neutral or zero radial rake at the highest cutting point, but a positive radial rake below this point, so as to simulate the action of the rhomboid insert in effecting positive radial rake cutting while the rounded or arcuate cutting edge also aids in expelling chips up an adjacent flute. As indicated, the radial rake changes drastically around the periphery of the cutting area of each round insert, but, as in the rhomboid insert, the curved cutting edge which progresses from a zero or neutral to a positive radial rake during cutting of the workpiece provides a shearing effect as it cuts to effect quiet operation requiring less power. Further, since the round or circular inserts have a relatively small cutting area, they produce smaller chips which contribute to its quiet performance.
Because of the curved cutting edges on both the circular and rhomboid inserts, actual cutting pressure causes a compression of these surfaces within the cutting area which is in effect larger than if the edge was straight which reduces tool wear significantly.
The circular or round inserts can optionally be mounted on the cutting area adjacent a flute at a positive axial rake angle, if desired, to obtain the benefits of axial rake cutting. The rhomboid and circular inserts include a strong included angular base which can be mounted in a 60.degree. V-shaped or conical pocket. In a preferred embodiment, the circular inserts include top and underside surfaces where the flat underside is joined to a side portion containing at least one pair of parallel oriented flats for distinct indexing within a tool pocket. The pockets include flat seating face and side support portions with parallel side flats for mating abutment with the inserts. The insert thus mounted may be indexed in sequence, depending on the number of flats present on the insert and pocket walls.
One advantage of this embodiment is that the abutting contact between the bottom face and side flats prevent loosening of the retaining screw due to the tangential force created by the shearing cutting action of the insert.
Another advantage is that the flat abutment between the insert and the recess, especially the bottom face, prevents the collection of chip material thereby facilitating rapid indexing of the inserts. The insert thus mounted results in an extremely strong mount for each insert and yet allows for accurate indexing to assure that a sharp edge is consistently presented to the workpiece.
The end mill can also be made with multiple, effective flutes or slots into which the chips can be driven and removed from, with the rows of cutting elements or inserts extending into adjacent flute areas being staggered in vertical extent up the cutting area of the tool so that the tool can effect cutting faster and cover all the areas of the workpiece, efficiently.